Polyurethanes are a versatile group of multi-phase, segmented polymers that have excellent mechanical and elastic properties, good hardness, high abrasion and chemical resistance.
In addition to elastomers, polyurethanes can also be produced as foams (rigid and flexible), adhesives, binders, coatings, and paints. Because of their unique properties, polyurethanes have found a wide variety of applications in the automotive, furniture, construction, and foot wear industries, as seating, exterior panels, structural foam, furniture, housing for electric equipment, shoe and boot soles, and refrigerator insulation.
Generally, polyurethane block co-polymers are comprised of a low glass transition or low melting “soft” segment and a rigid “hard” segment, which often has a glassy Tg, or crystalline melting point well above room temperature. The hard segment normally includes the connection of a diisocyanate (aromatic or aliphatic) and a low-molecular-weight diol or diamine, which is a chain extender. The combination of this soft polyol segment and hard segment generally forms an (AB)n type block co-polymer. By varying the structure, molecular weight of the segments, and the ratio of the soft to the hard segments, a broad range of physical properties can be obtained.
A urethane group is formed by the reaction between an alcohol and an isocyanate group. Thus, polyurethanes result from the reaction between an alcohol with two or more hydroxy groups (diol or polyol) and an isocyanate containing two or more isocyanate groups (diisocyanate or polyisocyanate).
Organotin compounds, especially dibutyltin dilaurate, are in widespread use as catalysts for the polyurethane reaction. Organotin compounds contain at least one direct bond between the tin and carbon atoms. In recent years there has been a great deal of public attention focused on the toxicological and environmental impacts or organotins, with special concern over the use of tributyl tin (TBT) due to its biocidal properties. Since 1988 the U.S. has banned the use of paints containing organotin compounds on water vessels that are shorter than 25 meters in length. The FDA has also placed limits on organotins to 3% in plastics that contact food (U.S. FDA 21CFR 178.2650 2000). In addition to concerns about the organotin content of various plastics, there is also the issue of worker exposure to much higher levels of these compounds when plant personnel handle the pure tin-containing additives. Furthermore, organotin residues have been found in articles, for example, in clothing manufactured from polyurethane fibers, thus exposing users of such articles to a risk of poisoning.
It would be desirable to find alternatives to organotin compounds as catalysts for use in polyurethane production.
U.S. Pat. No. 5,159,012 discloses a process for the manufacture of polyurethane elastomers from a reaction mixture which comprises a polyol, an isocyanate, water and a bismuth catalyst.
U.S. Pat. No. 5,587,448 concerns a reaction system for producing a polyurethane having an isocyanate index value of at least 100, and a catalyzed reaction mixture thereof, having a gel time between 5 and 60 minutes. The reaction system includes: (a) a first part comprising a polyisocyanate component; (b) a second part comprising: (i) a polyol component; (ii) a polyurethane catalyst comprising a bismuth/zinc polyurethane catalyst; and (iii) a molar excess of a complexing agent for the polyurethane catalyst, where the complexing agent is a mercaptan compound.
U.S. Pat. No. 4,804,691 discloses the preparation of a polyurethane using a catalyst selected from stannous octoate, a zinc compound, an aliphatic tertiary amine, dibutyltin diacetate or 1,4-diazabicyclo[2,2,2]octane.
Gorna et al (Journal of Polymer Science. Part A: Polymer Chemistry, Vol. 40, 156-170 (2002)) has described the synthesis of poly(ε-caprolactone) urethanes using poly(ε-caprolactonediols), diisocyanates and a range of catalyst systems. These include stannous octoate, dibutyltin dilaurate, magnesium, manganese and zinc.